The invention relates generally to image projection and more particularly, to image registration.
In some systems, an image projected on a screen must have a particular register, i.e., with a reference point or with another image or images projected on the same screen. As an example, where the images of two cathode ray tubes (CRT), each CRT image having a different color, are to be projected onto a single screen to form a single image having the color combination of both CRT images, the screen registration accuracy requirement for the two images can be very severe in order to avoid undesirable color fringing. The registration accuracy requirement may require registration within one picture element. In some systems where high resolution is required, a registration accuracy requirement of registration within one-fourth of a picture element may exist. For a CRT picture which is four inches in length in one dimension and has one thousand picture elements on that axis, a registration requirement of registration within 0.001 inch may exist.
Since image registration has been affected by component drift due to heat, warm-up times, age, use, etc., prior techniques invoked sophisticated designs of electrical, optical and mechanical components for stability in view of these factors. Even with these techniques, component drift was not compensated for and registration needed to be readjusted frequently. A further technique was the application of thermostatically controlled heaters to bring all components to a selected temperature which was above the ambient temperature. This technique still does not provide the registration accuracy required and the heat has an adverse effect on component reliability.